1. Field of the Invention
The present invention relates to a method for manufacturing a titania coated alumina fiber aggregate.
2. Description of the Related Art
Conventionally, it has been known that when pure aluminum or aluminum-based metal such as aluminum alloy is exposed to air under normal temperature, a surface of aluminum-based metal reacts with oxygen in air thus forming a natural oxide film on a surface thereof.
This natural oxide film is made of alumina (aluminum oxide) and exhibits advantageous effects such as heat resistance and corrosion resistance. However, only with the natural oxide film, it is difficult for aluminum-based metal to exhibit sufficient heat resistance and sufficient corrosion resistance not only under a high temperature condition but also under a normal environment. Further, to apply coating forming such as painting to aluminum-based metal, the formation of an oxide film having a larger thickness becomes necessary.
Accordingly, attempts have been made to achieve the enhancement of durability and heat resistance and the enhancement of working performance by imparting the two-layered structure to aluminum-based metal in which an artificial oxide film is formed by further oxidizing a lower layer of the natural oxide film.
As a method for forming the artificial oxide film which constitutes the second layer, a method which uses strong-acid solution and an oxidizing method which uses an anodization technique have been mainly known.
On the other hand, a fiber formed by aluminum-based metal (hereinafter simply referred to as aluminum fiber) exhibits an excellent strength and excellent formability compared to other metal fibers and, at the same time, the aluminum fiber possesses an extremely large film surface area and hence, the aluminum fiber is used as an industrial material or a carrier of a catalyst in broad fields.
Further, if a fiber which is formed by oxidizing a surface of the aluminum fiber with a large film thickness (hereinafter, referred to as alumina fiber) can be formed, dip coating formability is enhanced and hence, it is possible to produce a functional catalytic fiber which carries platinum or the like thereon or it is possible to apply photocatalytic titania coating which exhibits excellent durability to a surface of the alumina fiber. Accordingly, there has been a demand for the alumina fiber having the larger oxide film thickness.
However, in the treatment of the surface of the aluminum fiber by the method which oxidizes the surface of the aluminum fiber using a strong-acid solution, aluminum is resolved in the strong-acid solution and hence, it is difficult to manufacture the alumina fiber having the oxide film with a large film thickness.
Accordingly, there has been proposed a method which oxidizes an aluminum fiber by heating the aluminum fiber at a temperature of 100 to 400° C. in air without using a strong-acid solution (see JP-A-11-279843 (patent document 1), for example).